What should you do first?
Set the thermostat to Heat, raise the setpoint, check the filter, and confirm the furnace switch has not been turned off accidentally. If a CO alarm sounds, gas is suspected, the furnace repeatedly fails ignition, or the breaker trips, leave unsafe conditions and seek the appropriate emergency or repair help.
Safe observations
- Confirm Heat mode and a setpoint above room temperature
- Replace only a safely accessible, correctly sized filter
- Check that supply and return vents are open
- Note any diagnostic light pattern without removing panels
Warning signs and stop conditions
- CO alarm, gas odor, soot, or unusual combustion odor
- Repeated ignition attempts without heat
- Burning electrical smell or repeated breaker trip
- Loud boom, grinding, or metal-on-metal noise
- No heat during freezing weather
Possible causes
Airflow or limit shutdown
Restricted filters, blocked airflow, blower trouble, or overheating can cause a furnace safety limit to stop the heating cycle.
Ignition or fuel-control issue
Igniters, flame sensors, gas valves, pressure switches, venting, and controls can interrupt ignition. These are not DIY repairs.
Thermostat or electrical fault
A thermostat, transformer, control board, switch, wiring, or power problem can prevent a normal call for heat.
Professional diagnostic context
- Thermostat demand and control-board fault history
- Airflow, filter, blower, temperature rise, and safety limits
- Ignition sequence, flame proving, gas controls, and venting
- Electrical supply and component condition
- Heat exchanger condition when symptoms or age warrant inspection
What an estimate should explain
- The system type, capacity, age, and accessibility
- The failed component and whether compatible parts are available
- Diagnostic or service-call fees in the local market
- After-hours timing and current provider availability
- Refrigerant type, electrical work, and any additional testing required
Repair and replacement decisions
Minor controls and wear components may be reasonable to repair. A cracked heat exchanger, major reliability problems, or a high-cost failure on aging equipment calls for a careful safety and replacement comparison.
Questions and answers
Why is my furnace running but not heating?
The blower may be moving air while ignition, fuel, controls, airflow, or a safety limit prevents normal heat. Check settings and the filter, then arrange diagnosis if heat does not return.
What should I do if I smell gas?
Leave the home immediately. From outside, contact 911 or the local gas authority. Do not operate lights, switches, or phones inside the home.
Is a blinking furnace light useful?
Yes. Note the pattern and model information for the technician, but do not remove panels or bypass safety devices.
Can a furnace be repaired the same day?
It depends on the diagnosis, safe access, parts, and provider schedule. No one should promise completion before the problem is identified.